In a previous post ("Daily routine, with variety"), I discussed the need to keep variety in one's daily practice. It's far too simple to do the same thing day after day. Now that you've got a foundation and you've entered this middle stage, it's time to reassess your daily routine. (Of course, I advocate this reassessment on a daily basis, but this middle stage will require a higher level of assessment. You're in a new place now!)
With your solid foundation, you'll find that you don't have to do the same thing every day. There's nothing wrong with just playing your pieces, and you should not feel obligated to do mental work and landmark work every day. At the same time, you don't want to let your foundation slide, so you might want to alternate days of mental/landmark work with "performance" days. Whatever you do, let your planning and decision making be conscious, and avoid letting routine become a crutch.
You've only got so much time in your practice day, so it's still important to keep working with a timer and a practice log--especially when you've got to maintain your discipline with other pieces you're working on. It's so easy to get excited when you're finally able to play an entire piece after having spent so much time crawling through those early stages of memory! With that in mind, you need to remember that this middle stage is still a stage in the learning process.
I write these thoughts from experience. If you've read the early part of this blog, you'll remember that I had a number of significant lapse experiences when I returned to memorization practice after so many years away. What was missing from my work was a respect for this middle stage. I had quickly memorized my pieces (the early stage, but much too fast) and then naively paraded them out in performance without respecting the need to establish a firm post-memorized foundation.
After spending so much time to memorize your piece, the unfortunate fact is that you're just not there yet. If you embrace this stage as a learning stage, however, you will approach the concert stage with the confidence of knowing that you can, indeed, play by heart. Hang in there.